Are monoblocks better than stereo, with tube amps?

Manufacturers of high quality components equip their stereo amplifiers with sufficiently large enough power supplies to make it a non-issue.

I would be far more concerned with design, parts, and build quality.

Unless you are interested in "no holds barred" products, a stereo amplifier will do everything that a set of monoblocks will do.

There's no doubt that separating the power supplies and using a very short run of speaker cable with a long run of interconnect is a good thing. But there are some REALLY nice two channel amplifiers out there that are amazing.

Modulation of the power supplies is the issue here. Since almost nobody regulates the power supplies for the output section of their amplifiers, monoblocks will sound better for this reason alone. The result is greater authority, improved soundstage and a blacker background. BTW this has nothing to do with whether it is tube or solid state!

Stereo amplifiers can get around this problem by having separate power transformers and supplies for each channel. Then the only concern is proximity, which can be dealt with if wiring and layout concerns are addressed.

All other things equal, yes.

Not necessarily and in the end it is the sound that counts most. I would not let mono-bloc versus stereo make my decision.